Surveying, including electromagnetic surveying or seismic surveying, is used to perform characterization of subterranean elements in a subterranean structure. Examples of subterranean elements of interest include hydrocarbon-bearing reservoirs, fresh water aquifers, gas injection zones and other subterranean elements. Seismic surveying is performed by deploying seismic sources (e.g., air guns, vibrators, explosives, etc.) and seismic receivers (e.g., geophones, hydrophones, etc.). The seismic sources are used to produce seismic waves that are propagated into the subterranean structure, with some of the seismic waves reflected from subterranean elements of interest. The reflected seismic waves are received by the seismic receivers.
Similarly, electromagnetic (EM) surveying can use EM sources and receivers. One type of EM surveying is referred to as controlled source EM surveying (CSEM), in which an EM transmitter is used to generate EM signals that are propagated into the subterranean structure. Subterranean elements reflect the EM signals, with the reflected EM signals received by the EM receivers.
In a marine survey environment, survey hardware components (e.g., sources and receivers) can be towed through a body of water. To avoid interference from noise at the marine vessel that is towing the survey hardware components, the survey receivers on a streamer are typically located some minimum distance from the marine vessel. However, the survey sources are usually towed relatively close to the marine vessel to avoid possible entanglement issues between the towed source and receiver streamers. As a result, typical distances, in the in-line direction (which is the direction of movement of the marine vessel), is typically 50 to 200 meters away from the closest survey receiver of a streamer. Other reasons for towing a source close to the marine vessel is that a power cable that is used to power a source (such as a power cable to pump air to an air gun) may have a predefined maximum length. Also, the receivers are towed far away from the vessel so that vessel related noise (engine, crew, wake, etc.) is negligible.
The relatively large separation of survey source and survey receivers prevents measurements of a zero-offset or near-zero offset survey image. This can lead to reduced accuracy in processing of the survey data.